Like your first question, this one has no definite answer, because it doesn’t contain enough information. From what we’re told, it’s possible that all of the students who were taking typing were also taking accountancy; in that case there would be $400-150=250$ students not registered for either course, and no students registered for typing only. At the other extreme it’s possible that no student is taking both typing and accountancy; in that case there are $$400-(100+150)=400-250=150$$ students who are not registered for either course, and $100$ who are registered for typing only.
Where are you finding these questions? Are you sure that you’re giving us all of the information in the questions? Questions of this type usually do have enough information, so either you have a very odd source, or you’re missing something when you interpret the questions.